There is a desire amongst the operators of communication networks to transport both signaling and user traffic using Internet Protocol (IP) systems. This is because such systems can often make more efficient use of bandwidth than conventional systems such as those using circuit switched data links (e.g. TDM) and, perhaps more importantly, because the infrastructure associated with IP systems can be cheaper than the equipment required to implement conventional systems.
The term “IP backbone” is often used to denote an IP system used to interconnect various different types of subscriber access networks. The backbone can be thought of as providing “trunk” links between the access networks. The access networks themselves may themselves provide packet switched connections between subscribers and the IP backbone (e.g. in the case of GPRS access networks associated with cellular telephone networks), or may provide circuit switched connections (e.g in the case of Public Switched Telephone Networks (PSTN) and GSM and 3G voice networks).
As already mentioned, signalling and user data may be carried over an IP system such as an IP backbone: in the case of user data this is often referred to as Voice over IP (VoIP). However, a sometimes preferred mechanism is to carry user data over the IP backbone, and to carry signalling data over an SS7 network, the reason being that transmission over the IP network may be less reliable than that over an SS7 network. In order to allow for maximum flexibility, the Internet Engineering Task Force (IETF) has provided for the decomposition of gateways at network interfaces into Media Gateways (MGw:s) and Media Gateway Controllers (MGCs). The role of the MGw is to establish bearers for user data over the prescribed bearer network (e.g. the IP backbone). The role of the MGC is to handle call setup and control with peer MGCs (MGCs within the same communication plane), and to control one or more associated MGw:s so as to establish the bearers required for a negotiated call. A protocol known as the Gateway Control Protocol (GCP) has been defined for signalling over the interface between the media gateway controller and the media gateway.
FIG. 1 illustrates schematically a network belonging to a mobile operator. The network comprises an IP backbone 5, and a pair of WCDMA mobile access networks. A first of these networks 2 is located in a first geographical area (“Helsinki”) whilst a second of the access networks 4 is located within a second geographical area (“Turku”). Telephony traffic between the Helsinki mobile network and the Turku mobile network traverses an IP backbone 5, as does traffic exchanged between other pairs of mobile networks not shown in the Figure. The Helsinki area is also served by a first fixed line network or PSTN 1, whilst the Turku area is served by a second PSTN 3. The PSTN networks are coupled to respective mobile networks.
FIG. 1 illustrates by way of example a call originating in the Turlcu PSTN 3 which terminates at a mobile located within the Helsinki area. The call enters the mobile operator's network at the site Turlca, and Call servers (GMSC 6 and MSC 7) route the call from Turku to Helsinki and instruct the MGw:s at those sites to set up and handle the IP media bearers between the sites (in each direction, one of the MGw:s initiates a bearer and the other terminates the bearer). In this respect the call servers act as media gateway controllers for respective media gateways 8,9 (according to the IETF decomposition model). The IP backbone 5 contains a number of interconnected routers, although in FIG. 1 only the edge, or site routers 10,11 are shown.
The connectionless nature of the IP backbone 5 creates the problem of how to guarantee that there is enough capacity in the network to deliver a call between sites with a sufficient QoS. The call servers (MSC and GMSC) do not know the structure, capacity or traffic distribution within the backbone, and are typically set up to allow a certain volume of calls between the two sites. This does not guarantee however that there will always be capacity for this amount of calls between the sites. Routers and links in the IP backbone can be overloaded due to traffic between other sites sharing a common link. Routers and links in the backbone may also become non-operational due to failures, reducing the traffic carrying capacity of the backbone.
A protocol known as Resource Reservation Setup Protocol (RSVP) has been defined by the IETF for allowing resources to be reserved over an IP network. A MGw uses RSVP to establish a call connection having some specified Quality of Service (QoS). RSVP is generally applied on a per call basis, and results in the creation of state tables at network routers. In view of the large number of sessions which a router may be handling, the resulting increase on processing requirements is undesirable.
Call admission control mechanisms have been proposed which operate at the bearer level to limit the number and type of calls set up over the bearer network in an attempt to avoid overloading the bearer network. In a typical scenario, one of a pair of peer MGw:s (located in the same operating plane) is responsible for setting up a bi-directional bearer to the other MGw. MGw:s receive congestion reports from other MGw:s and make decisions on call admissibility accordingly.